This application claims the priority benefit of Japanese application serial no. 2001-190212, filed Jun. 22, 2001 and 2002-151022, filed on May 24, 2002.
1. Field of the Invention
This invention relates in general to a fixing device and an image forming apparatus, and more specifically relates to a curl straightening structure acting when a sheet is fixed.
2. Description of Related Art
In an image forming apparatus such as a copy machine, a printer, a facsimile or the like, generally after an electrostatic latent image, which is formed on a latent-image-support, is treated in a visualizing-image step of a developing process, the visible image is electrostatic transcribed to a recording medium such as a sheet of a recording paper, and the transcribed visible image is fixed thereon by heat in order to provide a so called copy or record.
A heat roller and a press roller, which face-to-face clamp a sheet-transporting path, are disposed on one of the devices used for fixing, and when these two rollers clamp and transport the sheet, a toner image is fixed by the heat roller with heat and pressure.
The fixing with the heat roller is popular nowadays, due to its advantages, such as achieving high efficiency and high speed in heat roller fixing, and achieving a high conductivity and stability in fixing efficiency, and having simple structures that are capable of using a sheet-transporting medium.
The fixed sheet is ejected to an ejecting tray by an ejecting unit with ejecting rollers, which are disposed near the heat roller and the press roller.
The toner-supporting-face becomes curved with respect to an imaginary center, resulting from the difference of the humidity between the heat roller and the inner face of the toner-supporting-face at the side contacting the heat roller. Then the curved face becomes curled. Once a sheet is curled, problems as follow appear.
In the case when ejecting a sheet in a horizontal direction, and the lower face of the sheet serves as the toner-supporting-face, the sheet will be ejected in a curled condition bending downward, and when it is ejected to an ejecting tray, a tip of the sheet will hang downward, and a jam is caused resulting from the tip protruding and hitting a transporting face of the ejecting tray such that the moving thereof is obstructed.
In order to straighten the curl of a sheet, some conventional structures have been developed as follows: one of the structures is made in such a way that the ejecting rollers, which clamp and transport a sheet, are made face-to-face, and the curvature radius of one of the ejecting rollers is much smaller than that of the other one, so that with the difference in curvature, the sheet will bend in a direction reverse to the direction of the curl during transporting (such as Japanese Laid-Open publication no. He5-162916);
Another one of the structures is made in such a way that one of the transporting guide units is extended in a direction reverse to the direction of the curl, making the sheet, which moves from the transporting guide unit to the other one within the transporting path, imitate the shape of the transporting guide unit, to straighten the curl;
Another one of the structures is made in a way that eliminates the curl with the orientation by making the sheet move along the guide face of the transporting guide;
Another one of the structures is made in such a way that by changing the position where the sheet is clamped and transported in, the transporting direction is changed in order to straighten the curl (such as Japan Laid-Open publication no. He7-121039, He7-285721, He8-137309 and He8-290857);
Another one of the structures in made in such a way that, in order to straighten the curl by using the circumference of the ejecting rollers, make a pair of other rollers contact along the circumferences of the ejecting rollers, then the sheet is clamped and transported by the ejecting rollers and the pair of other rollers, so that the curl of the sheet can be straightened along the circumferences of the ejecting rollers (such as Japanese Patent no. 2547722).
A face of a sheet, which supports the toner image, bends towards the side and causes a curl resulting from the difference of the humidity between the face of the sheet supporting the toner and the face not supporting the toner. Therefore, the conventional curl straightening structure eliminates the curl by loading in a direction reverse to them direction of the curl, to turn the sheet back.
Among the structures that bend the sheet in a direction reverse to the direction of the curl, in the case when using a non-moving guide unit which constructs a transporting path, the tip of the sheet will contact with the guide unit again after it has been straightened during moving, so the tip of the sheet bent reverse to the curl will easily encounter a large resistance during moving, and some failures will happen in transporting.
To solve this problem, the Japanese Patent no. 547722 discloses that, in the case when transporting rollers are used in the sheet-transporting path to straighten the curl, it is necessary to keep the transporting rollers in contact with the ejecting rollers, and it is further necessary to lead the sheet toward the contacting position of the ejecting rollers and the transporting rollers, and therefore the structures are complicated. Especially when the sheet enters the contact position on the ejecting rollers and the transporting rollers, the sheet is separated from the transporting guide unit, and therefore, it is difficult to lead the sheet to the contacting position, so some failures easily happen in transporting as in the case described above.
The conventional structures to straighten the curl in other ways are as follows: one of the structures is that, when clamping and transporting the sheet, which is transported from a fixing device, by a pair of rollers opposed along the transporting path, the sheet is forced with an extension due to the difference in transporting velocity between the rollers. In this condition, the sheet is pressed in a direction reverse to the curl direction with another middle roller, which is disposed between the rollers, to straighten the curl (such as Japanese Laid-Open publication no. He8-227188);
Another one of the structures is that the sheet ejected from the fixing device is clamped by the ejecting rollers and a pair of other rollers, which are disposed along the circumferences of the ejecting rollers of the ejecting unit, and the sheet is turned around in a direction reverse to the direction of the curl with the roller upstream of moving sheet, and then after making the sheet move imitating the circumferences of the ejecting rollers, eject the sheet during it is clamped again by the ejecting rollers and the other rollers (such as the Japanese Laid-Open publication no. 2000-122351).
However, these curl straightening structures disclosed in publications have problems as follows.
For straightening the curl, it is necessary to bend the sheet in a direction reverse to the curl, and it is also necessary to make the sheet have no inclination to curl again after it has been straightened. The flexural rigidity, the so called strength of the waist, is important. However, the publications described above disclose the curl straightening, but disclose nothing about structures preventing the sheet from re-curling during transport after the sheet is curl-straightened. Especially, as in Japanese Laid-Open publications no. He8-227188 and 2000-1223551, they relate nothing about turning around the sheet to the direction reverse to the curl, and they only disclose bending the sheet in a direction the same as the re-curl with the transporting unit during the ejecting of the sheet.
To solve the problems of the conventional fixing device described above, the aim of the present invention is to provide a fixing device and an image forming apparatus, which are constructed in such a way that can easily straighten the curl without increasing cost and can prevent sheet re-curling from being generated after the sheet has been straightened.
The fixing device of the present invention comprises: a heat unit and a press unit, disposing a sheet opposite to a midway position of the sheet ejecting/transporting path, wherein the sheet supports a toner image; and an ejecting unit, wherein the toner image is fixed on the sheet by heat and pressure of the heat unit and the press unit, then the sheet is transported to the ejecting unit, and the sheet is ejected by the ejecting unit in a way that makes the sheet keep in a condition turned back reversed to a bending direction generated when the sheet is sent out from the heat unit and the press unit.
The fixing device of the present invention further features that on the sheet ejecting/transporting path, a position opposite to the heat unit and the press unit serves as a starting point, and a position, which is set along the bend direction, and on which the ejecting unit is disposed in a way that turning back the sheet ejecting/transporting path reverse to the bending direction, serves as an ending point, and the transporting guide unit is set on a position changing the transporting path from the bending direction to a reverse direction, and the sheet is kept in a condition with turning back when moving by the transporting guide unit in a way that the sheet moves to the starting point of the sheet ejecting/transporting path, the transporting guide unit and the ending point in sequence.
The fixing device of the present invention further has the feature that the sheet is ejected by the ejecting unit in a condition turning back reversed to the bending direction, and the transporting guide unit is set to turn back the sheet in such a way that the transporting guide unit is positioned so that it does not cross a connecting line from a clamping position of the sheet on the ejecting unit.
The fixing device of the present invention further has the feature that the transporting guide unit is set close to or apart from the sheet ejecting/transporting path.
The fixing device of the present invention further has the feature that the transporting guide unit is forced toward the sheet ejecting/transporting path by an elastomer, and moves to a position not crossing a connecting line from a sheet clamping position on the ejecting unit when the transporting guide unit faces the sheet.
The fixing device of the present invention further has the feature that the transporting guide unit is constructed with small sliding-friction resistance with respect to the sheet.
The fixing device of the present invention further has the feature that the transporting guide unit is constructed with a roller.
The fixing device of the present invention further has the feature that the transporting guide unit is constructed with a guide plate with a rib shape having a face contacting with the sheet with low friction resistance.
The fixing device of the present invention further has the feature that the plurality of ejecting units is disposed to move the sheet in a condition turning back the sheet reverse to a bending direction and perpendicular to an ejecting direction of the sheet, and increase a flexural rigidity of the sheet, which is guided by the transporting guide unit, in a way to make the sheet have a habit of bending reversed to a curl direction in the ejecting direction.
The ejecting unit clamps the sheet ejecting/transporting path, and has a first ejecting unit and a second ejecting unit, wherein the first ejecting unit, which faces a toner-image-supporting face of the sheet, has a length longer than that of the second ejecting unit in a direction perpendicular to the ejecting direction, and a flange portion formed at each of the ends of the first portion has a diameter larger than that on the other circumference of the first ejecting unit, and the second ejecting unit is oppositely in contact with the first ejecting unit between the flange portions.
The image forming apparatus of the present invention comprises the fixing device described as above.